Preimplantation genetic screening (PGS) is a procedure in which one or more cells are biopsied from an embryo and analyzed to determine the karyotype of the embryo. The biopsy typically is performed by an embryologist, who places the cell(s) in a tube with 2-3 ul of liquid before freezing and delivering to a laboratory for testing. The laboratory receives the frozen liquid containing the biopsied cell(s), and performs a lysis procedure to liberate the nucleic acid. The lysis can be performed in several different ways, but the most common technique uses an alkaline solution, optionally in the presence of elevated temperature, to lyse the cell(s) and potentially denature and/or shear the nucleic acid for downstream analysis.
By profiling embryos for certain genetic disorders, a prospective parent may select embryos that lack the disorder, have an increased chance of successful pregnancy, have a lower predisposition to cancer, and a variety of other considerations. Preimplantation genetic diagnosis is used to select desired oocytes or embryos from a group of oocytes or fertilized embryos, some of which may be at risk, rather than assaying for a specific disease or condition. Preimplantation genetic screening can also be used to select embryos that have a euploid chromosome complement (22 pairs of autosomes and 2 sex chromosomes), or that lack finer-scale genomic rearrangements including translocations or sub-chromosomal gains or losses, thereby increasing the chance that the embryo will successfully implant, develop to term, and result in a healthy live birth, and decreasing the potential desire for selective pregnancy termination.
Preimplantation genetic screening may involve removing multiple eggs from a donor, fertilizing them to produce embryos, analyzing the embryos, and selecting an embryo that meets certain desired characteristics. To analyze a preimplanted embryo or blastocyst, only a small amount of cellular material can generally be biopsied. Because the embryo or blastocyst may have only a few dozen cells altogether, a PGS assay may involve biopsying only a single cell. Generally, the cell or cells are biopsied and analyzed to determine the karyotype of the embryo.